Large air gap motors have previously been proposed, for example, as shown in U.S. Pat. Nos. 3,911,299; 4,130,769; 4,259,604; 4,412,146; 4,445,061; and 4,645,961. U.S. Pat. No. 3,911,299 utilized a slotless core in the stator made from conventional stator laminations which were flat sheets in a plane perpendicular to the rotor axis. Also, this patent recognized the problem of heat from the stator winding getting into the bearings, and proposed the use of an asbestos heat shield on the inside of the stator winding.
U.S. Pat. No. 4,130,769 utilized pancake coils of a single thickness in the armature winding, with these coils arcuately displaced and partially overlapped.
U.S. Pat. No. 4,412,146 utilized a stationary permanent magnet and a stationary outer core with a rotating winding. In U.S. Pat. No. 4,259,604, the design was also one with a rotating coil winding.
U.S. Pat. No. 4,445,061 suggests the use of a ferrite material as the slotless stator core.
U.S. Pat. No. 4,645,961 utilizes a slotless stator, with the armature winding provided as a printed circuit on a flexible insulator material.
U.S. Pat. No. 4,255,684 utilizes a stator core which was originally wound in a spiral, but then was cut apart to form two C-shaped stator yokes.
U.S. Pat. No. 4,395,815 utilized a laminated structure for a stator which was a strip of magnetic material wound on edge, but this had a tendency to buckle as it was wound.
U.S. Pat. No. 4,644,208 utilized a spiral ribbon of magnetic material as a stator yoke; however, this was not in a slotless core configuration.
U.S. Pat. No. 4,292,558 utilized pancake coils in a large machine, such as a turbo generator. These pancake coils were arcuately offset and partially overlapping, so that one side of the coil was not the same distance from the axis as the other side.